Place the potatoes in a medium-size pot, cover them with cold water and bring to a boil. Salt the water and cook potatoes until fork tender, about 12-15 minutes. Drain the potatoes and allow them to cool slightly. Mash them with the milk then stir in the egg and horseradish. Season with salt and freshly ground black pepper and set aside.

While the potatoes are cooking, make the filling: Heat the EVOO in a large, deep skillet over medium-high heat until it shimmers. Add the meat and brown it well, about 5 minutes. Add the vegetables and bay leaf to meat, season with salt and pepper and cook until the vegetables are softened, 8-10 minutes. Sprinkle the flour around the pan, stir and cook for about 1 minute. Add the stock and bring up to a simmer to thicken the gravy, 3-4 minutes. Discard the bay leaf and season the gravy with Worcestershire sauce, salt and pepper.

Turn broiler on and place rack in the middle of the oven.

Transfer the filling to a medium-size casserole or baking dish and top it with heaping spoonfuls of the horseradish mashed potatoes so they completely cover it. Brown the potatoes under the broiler, about 2 minutes, garnish with chives and serve.